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On 2018-02-25 18:15, Ludwig Schweinfurth wrote:
> When you drop bombs using an offset aim point heading error is very 
detrimental and gets worse with distance to the aimpoint.  One way we 
corrected for that was to move the crosshairs from one aimpoint to the other 
and if they fell off in a perpendicular dirrection it was attributed to 
heading error.  We could dial out the error on the gyro if we had time. The 
other interesting radar navigation technique was called target timing wind.  
We could aim at a target, wait a few minutes for the crosshairs to drift off, 
put the crosshairs back on the target, and the computer (analog) would give 
us the true wind dirrection and velocity.

The ASB-9A bombing-navigation system of the B-52H had a "memory point"
mode which performed that last function. It was a variant of "track"
mode: the system drove the radar crosshairs at a velocity corresponding
to ground speed. An error in wind velocity caused them to drift off
target. With the hand control (joystick) you could correct the crosshair
placement. In memory point mode the hand control inputs also drove the
wind east and wind north counters (similar to mechanical odometers) to
update the wind components.

Regarding offset aim points, we ought to explain that in many cases the
target of a bomb is difficult or impossible to positively identify on
radar. Therefore the radar crosshairs are placed not on the target, but
on a suitable offset aim point. Of course the coordinates of the OAP
with respect to the target must be known from intelligence analysis, and
set into the bombing system.

Heading error affected the accuracy of offset bombing and as mentioned
there were techniques to compensate. But I heard from one radar
navigator that the transition from astrotracker to the SPN/GEANS
inertial nav system in the 1980s practically eliminated heading error.

The SPN/GEANS gyro is a hollow beryllium sphere about the size of a
ping-pong ball, suspended in vacuum by electrostatic fields. (The vacuum
chamber is only a few thousandths of an inch larger than the sphere.) An
optical sensor at the top of the chamber observes a mark on the north
pole of the ball, thereby providing feedback so the gimbal torque motors
can keep the gyro and accelerometer platform aligned to its spin axis.

As I recall, operating speed of the ball was about 650 rps. The spin
electronics shut off when that speed was attained, and during navigation
it coasted. When I went to school on this system the instructor said
several years would go by before it slowed down too much. But since the
ball needed power to remain suspended, it was brought to a stop when the
SPN/GEANS shut down. A battery provided backup power, since loss of
suspension voltage would instantly destroy the ball.

I heard wild stories about technicians opening up the ceramic envelope
and finding nothing but dust, but I suspect these were myths. We who
worked on the system in the field never even glimpsed the gimbals. You
had to be in a clean room to open the inertial measurement unit. In
school we did get to play with a beryllium ball which had been rejected
at the factory. It was a minutely prolate spheroid, we were told, to
compensate for the expansion at the equator at operating speed. Due to
its internal mass distribution, when set on a table and spun it would
immediately upright itself.

With the old ASB-9A, when we ran up the system it was sufficient to
enter 47 57 N 097 24 W as the aircraft position. (My coordinates may be
wrong — it's been 35 years.) But new digital avionics with SPN/GEANS
required a cheat sheet with a list of coordinates for each parking spot.
There was even a survey mark in the shop floor beneath the test set.

   

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